Electrostatic coating of phosphors on glass substrates, for the ultimate purpose of lamp making, is discussed in the patent literature. Both bulbous and linear glass shapes have been coated by this method. Electrostatic coating processes are characterized by the following key steps: (1) feeding of powder to a carrier gas stream; (2) transport of the powder laden gas to a high voltage probe; (3) charging of the powder in the corona surrounding the probe; (4) transporting the charged powder particles in the carrier gas stream to the vicinity of a substrate maintained at a suitable temperature preferably above ambient, and at an electrical potential suitably different from the probe potential thereby creating an electric field such that the charged particles may migrate, under the action of this electric field, towards the substrate; (5) depositing the charged particles on the substrate; (6) thermally treating the coated substrate to bind the coating to the substrate.
This invention advances the state-of-the-art in electrostatic coating by making improvements in step (4). In particular, this disclosure describes a means for providing an optimum electric field configuration and strength for the electrostatic coating of phosphors on asymmetrical glass substrates in general, and compact fluorescent lamp glass in particular, while maintaining the temperature of the glass substrate within an optimum range by means of conformal heating.
The configuration of the electric field around a glass substrate controls the distribution of the coating and the overall extent of surface coverage of the substrate. The electric field strength influences the time scale of radial motion of the charged particles to the substrate relative to the time scale of convective axial motion of the particles due to the drag force exerted by the carrier gas. The smaller the radial time scale relative to the axial time scale the shorter is the axial distance traveled by the charged particles after leaving the probe, before they are deposited on the substrate. This promotes quick deposition of the charged phosphor particles soon after they exit the corona region.
In addition, published literature shows that q/m or the charge to mass ratio of a particle is a function of the electric field strength to which the particle is subjected. In particular q/m is proportional to the electric field strength, E, as proven in the Pauthenier Equation [see "Powder Coating Technology" by J. F. Hughes in Journal of Electrostatics, 23, 3 (1989)]. Published literature (ibid.) also indicates that q/m is one of the most important parameters governing the quality of electrostatic coating. A low value of q/m implies poor charging of the powder with subsequent poor adhesion and loss of material due to overspray. The importance of the electric field strength cannot, therefore, be overemphasized.
The temperature of the glass substrate influences its electrical conductivity. In particular, the higher the glass temperature the higher its conductivity. It is worth noting that the change in conductivity with temperature is non-linear. It is necessary that the glass substrate be sufficiently conductive such that the charged particles migrating towards it may induce an opposite polarity mirror charge on the near surface of the glass. This mirror charge is necessary for the initial adhesion of the coating.
Too high a glass conductivity is not, however, desirable. Electrical conductivity in most glasses is ionic in nature, with the sodium ion being responsible for the lion's share of the current. At high temperatures, large amounts of sodium are prone to diffuse out of the glass into the coating. Presence of sodium is detrimental to phosphors in that it leads to lumen losses with time in the finished lamp. There is, therefore, an optimum temperature range for each type of glass substrate. By type, we refer here to the chemical composition of the glass. Glasses which have higher sodium content are more prone to this diffusion problem than glasses with lower contents of alkali. For a large variety of glasses, the logarithm of the resistivity varies linearly with the reciprocal of the absolute temperature. This relation for common fluorescent lamp glass may be approximated by the relation: log .rho.=-2.1+4.44*(1000/T) where .rho. is the resistivity in .OMEGA..cm and T is the absolute temperature. While .rho. changes by about a factor of thirteen between 150.degree. C. and 200.degree. C., the change between 200.degree. C. and 250.degree. C. is about a factor of eight. The mathematical relationship between .rho. and T was obtained by linear regression of data presented in Glass Engineering Handbook, 3rd Edition, George W. McLellan and E. B. Shand, McGraw Hill, 1984.
Present methods of electrostatic coating of phosphors on glass substrates are concentrated to bulbous shaped glass typically used for incandescent and high intensity discharge lamps and cylindrical shaped glass used for large linear fluorescent lamps. Examples of recent patents in this field are U.S. Pat. No. 5,032,420 for Cd free yellow incandescent bug lights and U.S. Pat. No. 4,914,723 for a linear fluorescent lamp. It is noted that both bulbous shaped glass for incandescent lamps and cylindrical glass for linear fluorescent lamps are symmetrical shapes which can easily be rotated about their axis. This makes it possible to heat these shapes by a flame without the adverse possibility of softening because the constant rotation of the glass prevents local overheating. Flames are, therefore, the present method of choice in the electrostatic coating of such symmetrical glass shapes. A U-shaped piece of glass like a compact fluorescent lamp glass is, however, asymmetrical. This makes it very difficult to rotate this shape, as a result of which the method of flame heating is rather impractical for compact fluorescent lamp glass.
A flame always contains charged species, and the use of a flame in the electrostatic coating of symmetrical glass shapes also provides an almost zero potential to the glass. For all practical purposes the substrate is, therefore, at ground potential in contrast to the higher (in magnitude) potential associated with the charging probe. This generates the electric field for the migration of the charged phosphor particles to the substrate. In the electrostatic coating of symmetrical glass shapes, therefore, the flame method serves to both heat the glass and provide the electric field. The ability to control the electric field strength using the flame approach is minimal. In addition, unless the flame drapes the glass uniformly, there is a possibility that the control over the configuration of the electric field may also be deficient. Since the use of flames on asymmetrical glass shapes is a problem, neither heating nor electric field generation is practical for asymmetrical glass shapes using the flame approach.
In an alternate approach adopted in the electrostatic coating of phosphors on symmetrical glass shapes, the glass is preheated by some suitable means and rotated about its axis of symmetry while an electrically conductive material touches the exterior of the glass. A metallic brush is frequently used and serves as a path to ground for the charge carried by the phosphor particles to the glass substrate. While this technique provides an electric field, substrate temperature control is not available and the glass temperature is likely to change over the course of the coating cycle. In addition, the control over the configuration and strength of the electric field is barely satisfactory.
It is apparent, therefore, that existing means of generation and control of electric field strength, electric field configuration and substrate heating as applied to the electrostatic coating of asymmetrical glass substrates are deficient. In light of these deficiencies, a new method is proposed whose operation will be clear from the description that follows.